This invention relates to a mold for microlens, a process for its production and microlens. Small lenses with diameters less than a few millimeters are referred to as microlenses. Several of them may be formed in a neatly arranged array or they may be used singly, as disclosed in Optoelectronics Technical Dictionary (edited by Shunichi Tanaka, et al. and published by Ohm Publishing Co. (1996)). This invention relates to a mold that may be used for the production of such a microlens, a process for its production and microlens.
Throughout herein, the term “microlens” may be used to indicate both a single microlens and a microlens array, especially they are not particularly distinguished.
Lenses for cameras and optical microscopes are produced by polishing an optical material or by obtaining a mold for a molding process. These production methods for ordinary lenses cannot be used for the production of microlenses with a small size. For this reason, different methods as explained below are used for the production of microlenses. Since microlenses are divided into the refraction type and the diffraction type, these types are separately explained.
Microlenses of the diffraction type are produced principally by using a production technology for semiconductor integrated circuits. In other words, technologies such as photolithography and electron beam lithography are used for the production, but the required accuracy in superposition is severe with photolithography, and electron beam lithography is disadvantageous in that patterning in a special shape such as a circle is required and that the productivity is low. Microlenses of the diffraction type are also disadvantageous in that chromatic aberration is large and that the efficiency of light convergence is low.
The ion exchange method and the reflow method are representative production methods for microlenses of the refraction type. When a microlens array of the refraction type is produced by the ion exchange method, a metallic film with openings at the positions of microlenses is formed on a glass substrate and this is immersed in molten salt in this condition. If thallium sulfate is used as the molten salt, the glass substrate is immersed in it for over 100 hours. Not only is this production method time-consuming, but it is also difficult to obtain an ideal distribution of refraction index by this method. In particular, this method is disadvantageous in that the aberration becomes large at the circumference of the microlenses.
The reflow method includes the following four steps, as described in FIG. 23A (See “Applied Optics”, Vol. 27, No. 7, pages 1281-1284 (1988)): (a) the step of forming a thin aluminum film over a quartz substrate and providing the aluminum film with openings of diameter 15 μm; (b) the step of forming circular pedestals of diameter 30 μm over these openings, these pedestals being processed so as to be insoluble in the solvent and stable at temperatures over 180° C.; (c) the step of forming circular columns with diameter 25 μm and height 12 μm with photoresist on these circular pedestals; and (d) the step of heating for 15 minutes at 140° C. to produce an array of spherically shaped microlenses. The transformation into the spherical shapes by this reflow method is based on the principle of surface energy being minimized by surface tension.
Accurate measurements cannot be obtained by this reflow method, however, because the surface tension is easily affected by external disturbances. Thus, fluctuations are likely to result in the optical characteristics of the lenses. When a lens array is produced, furthermore, the spheres must be prevented from mutually contacting such that the surface tension will not deform them. Thus, flat portions (or non-lens portions) are formed between mutually adjacent spheres, as shown in FIG. 23B. When the product is used as a lens array, the portion of the light that passes through such flat portions becomes the stray light, causing an increase in noise, reducing the efficiency in the passage of light and generating cross talks. Since it is difficult to eliminate such flat portions, the ratio {(the array area)−(the flat area within the array area)}/(the surface area) is referred to as the “fill factor” and is considered one of the indicators for the evaluation of the characteristics of a microlens array.
As explained above, microlenses of the refraction type produced by conventional methods can at best be called nearly spherical. Although they can converge light and improve the efficiency of light transmittance, the convergence of light is not like that by a lens of the normal size. In other words, there is currently no microlens of the refraction type having high optical characteristics.